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<?xml version="1.0" encoding="utf-8" ?>
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<title>pointee and indirect_reference</title>
<meta name="author" content="David Abrahams" />
<meta name="organization" content="Boost Consulting" />
<meta name="date" content="2006-09-11" />
<meta name="copyright" content="Copyright David Abrahams 2004." />
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<body>
<div class="document" id="pointee-and-indirect-reference">
<h1 class="title"><tt class="docutils literal"><span class="pre">pointee</span></tt> and <tt class="docutils literal"><span class="pre">indirect_reference</span></tt></h1>
<table class="docinfo" frame="void" rules="none">
<col class="docinfo-name" />
<col class="docinfo-content" />
<tbody valign="top">
<tr><th class="docinfo-name">Author:</th>
<td>David Abrahams</td></tr>
<tr><th class="docinfo-name">Contact:</th>
<td><a class="first last reference external" href="mailto:dave&#64;boost-consulting.com">dave&#64;boost-consulting.com</a></td></tr>
<tr><th class="docinfo-name">Organization:</th>
<td><a class="first last reference external" href="http://www.boost-consulting.com">Boost Consulting</a></td></tr>
<tr><th class="docinfo-name">Date:</th>
<td>2006-09-11</td></tr>
<tr><th class="docinfo-name">Copyright:</th>
<td>Copyright David Abrahams 2004.</td></tr>
</tbody>
</table>
<!-- Distributed under the Boost -->
<!-- Software License, Version 1.0. (See accompanying -->
<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">abstract:</th><td class="field-body">Provides the capability to deduce the referent types of
pointers, smart pointers and iterators in generic code.</td>
</tr>
</tbody>
</table>
<div class="section" id="overview">
<h1>Overview</h1>
<p>Have you ever wanted to write a generic function that can operate
on any kind of dereferenceable object? If you have, you've
probably run into the problem of how to determine the type that the
object &quot;points at&quot;:</p>
<pre class="literal-block">
template &lt;class Dereferenceable&gt;
void f(Dereferenceable p)
{
<em>what-goes-here?</em> value = *p;
...
}
</pre>
<div class="section" id="pointee">
<h2><tt class="docutils literal"><span class="pre">pointee</span></tt></h2>
<p>It turns out to be impossible to come up with a fully-general
algorithm to do determine <em>what-goes-here</em> directly, but it is
possible to require that <tt class="docutils literal"><span class="pre">pointee&lt;Dereferenceable&gt;::type</span></tt> is
correct. Naturally, <tt class="docutils literal"><span class="pre">pointee</span></tt> has the same difficulty: it can't
determine the appropriate <tt class="docutils literal"><span class="pre">::type</span></tt> reliably for all
<tt class="docutils literal"><span class="pre">Dereferenceable</span></tt>s, but it makes very good guesses (it works
for all pointers, standard and boost smart pointers, and
iterators), and when it guesses wrongly, it can be specialized as
necessary:</p>
<pre class="literal-block">
namespace boost
{
template &lt;class T&gt;
struct pointee&lt;third_party_lib::smart_pointer&lt;T&gt; &gt;
{
typedef T type;
};
}
</pre>
</div>
<div class="section" id="indirect-reference">
<h2><tt class="docutils literal"><span class="pre">indirect_reference</span></tt></h2>
<p><tt class="docutils literal"><span class="pre">indirect_reference&lt;T&gt;::type</span></tt> is rather more specialized than
<tt class="docutils literal"><span class="pre">pointee</span></tt>, and is meant to be used to forward the result of
dereferencing an object of its argument type. Most dereferenceable
types just return a reference to their pointee, but some return
proxy references or return the pointee by value. When that
information is needed, call on <tt class="docutils literal"><span class="pre">indirect_reference</span></tt>.</p>
<p>Both of these templates are essential to the correct functioning of
<a class="reference external" href="indirect_iterator.html"><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt></a>.</p>
</div>
</div>
<div class="section" id="reference">
<h1>Reference</h1>
<div class="section" id="id1">
<h2><tt class="docutils literal"><span class="pre">pointee</span></tt></h2>
<!-- Copyright David Abrahams 2004. Use, modification and distribution is -->
<!-- subject to the Boost Software License, Version 1.0. (See accompanying -->
<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
<pre class="literal-block">
template &lt;class Dereferenceable&gt;
struct pointee
{
typedef /* see below */ type;
};
</pre>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Requires:</th><td class="field-body">For an object <tt class="docutils literal"><span class="pre">x</span></tt> of type <tt class="docutils literal"><span class="pre">Dereferenceable</span></tt>, <tt class="docutils literal"><span class="pre">*x</span></tt>
is well-formed. If <tt class="docutils literal"><span class="pre">++x</span></tt> is ill-formed it shall neither be
ambiguous nor shall it violate access control, and
<tt class="docutils literal"><span class="pre">Dereferenceable::element_type</span></tt> shall be an accessible type.
Otherwise <tt class="docutils literal"><span class="pre">iterator_traits&lt;Dereferenceable&gt;::value_type</span></tt> shall
be well formed. [Note: These requirements need not apply to
explicit or partial specializations of <tt class="docutils literal"><span class="pre">pointee</span></tt>]</td>
</tr>
</tbody>
</table>
<p><tt class="docutils literal"><span class="pre">type</span></tt> is determined according to the following algorithm, where
<tt class="docutils literal"><span class="pre">x</span></tt> is an object of type <tt class="docutils literal"><span class="pre">Dereferenceable</span></tt>:</p>
<pre class="literal-block">
if ( ++x is ill-formed )
{
return ``Dereferenceable::element_type``
}
else if (``*x`` is a mutable reference to
std::iterator_traits&lt;Dereferenceable&gt;::value_type)
{
return iterator_traits&lt;Dereferenceable&gt;::value_type
}
else
{
return iterator_traits&lt;Dereferenceable&gt;::value_type const
}
</pre>
</div>
<div class="section" id="id2">
<h2><tt class="docutils literal"><span class="pre">indirect_reference</span></tt></h2>
<!-- Copyright David Abrahams 2004. Use, modification and distribution is -->
<!-- subject to the Boost Software License, Version 1.0. (See accompanying -->
<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
<pre class="literal-block">
template &lt;class Dereferenceable&gt;
struct indirect_reference
{
typedef /* see below */ type;
};
</pre>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Requires:</th><td class="field-body">For an object <tt class="docutils literal"><span class="pre">x</span></tt> of type <tt class="docutils literal"><span class="pre">Dereferenceable</span></tt>, <tt class="docutils literal"><span class="pre">*x</span></tt>
is well-formed. If <tt class="docutils literal"><span class="pre">++x</span></tt> is ill-formed it shall neither be
ambiguous nor shall it violate access control, and
<tt class="docutils literal"><span class="pre">pointee&lt;Dereferenceable&gt;::type&amp;</span></tt> shall be well-formed.
Otherwise <tt class="docutils literal"><span class="pre">iterator_traits&lt;Dereferenceable&gt;::reference</span></tt> shall
be well formed. [Note: These requirements need not apply to
explicit or partial specializations of <tt class="docutils literal"><span class="pre">indirect_reference</span></tt>]</td>
</tr>
</tbody>
</table>
<p><tt class="docutils literal"><span class="pre">type</span></tt> is determined according to the following algorithm, where
<tt class="docutils literal"><span class="pre">x</span></tt> is an object of type <tt class="docutils literal"><span class="pre">Dereferenceable</span></tt>:</p>
<pre class="literal-block">
if ( ++x is ill-formed )
return ``pointee&lt;Dereferenceable&gt;::type&amp;``
else
std::iterator_traits&lt;Dereferenceable&gt;::reference
</pre>
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